Joint fluid Gram stain

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Definition

Joint fluid Gram stain is a laboratory test to identify bacteria in a sample of joint fluid using a special series of stains (colors). The Gram stain method is one of the most commonly used methods to rapidly identify the cause of bacterial infections.

Alternative Names

Gram stain of joint fluid

How the Test is Performed

A sample of joint fluid is needed. This may be done in a health care provider's office using a needle, or during an operating room procedure. Removing the sample is called joint fluid aspiration.

The fluid sample is sent to a lab where a small drop is spread in a very thin layer onto a microscope slide. This is called a smear. Several different colored stains are applied to the sample. The laboratory personnel will look at the stained smear under a microscope to see if bacteria are present. The color, size, and shape of the cells help identify the bacteria.

How to Prepare for the Test

Your provider will tell you how to prepare for the procedure. No special preparation is needed. But, tell your provider if you're taking a blood thinner, such as aspirin, warfarin (Coumadin) or clopidogrel (Plavix). These medicines can affect test results or your ability to take the test.

How the Test will Feel

Sometimes, the provider will first inject numbing medicine into the skin with a small needle, which will sting. A larger needle is then used to draw out the synovial fluid.

This test may also cause some discomfort if the tip of the needle touches bone. The procedure usually lasts less than 1 to 2 minutes.

Why the Test is Performed

The test is performed when there is unexplained swelling, joint pain, and inflammation of a joint, or to check for suspected joint infection.

Normal Results

A normal result means no bacteria are present on the Gram stain.

What Abnormal Results Mean

Abnormal results mean bacteria were seen on the Gram stain. This may be a sign of a joint infection, for example, gonococcal arthritis or arthritis due to bacteria called Staphylococcus aureus.

Risks

Risks of this test include:

Infection of the joint -- unusual, but more common with repeated aspirations